1. Field of the Invention
The invention relates to a milling device having a milling roller, a milling ring and a drive, wherein said milling roller has a diameter substantially equal to or larger than an inner radius of said milling ring and is running along a substantially cylindrical inner surface of said milling ring. Such a milling device is e.g. suited for milling stones to sand.
2. Description of the Prior Art
A milling device of this kind is e.g. described in WO 87/06500 and FR-349 886. Several advantages result from using a mill with such a large milling roller. For one, the rolling friction is reduced considerably when compared to mills having a plurality of smaller rolls arranged in a single plane such as they are e.g. described in EP 0 102 645. Furthermore, mechanical set-up becomes easier because a smaller number of bearings is required, and the large, heavy mass of the milling roller is not easily deflected by hard foreign bodies, such as metals, in the material to be milled.
In conventional milling devices having a single large roller, the milling roller is directly driven for rotation about its central roller axis, which causes the roller to run along the cylindrical inner surface of the milling ring.
Such mills show various disadvantages. Especially, it has been found that the frequency of the center of mass of the milling roller circling the center of mass of the milling ring--the running frequency--depends strongly on the diameter of the milling roller. This effect causes a change in the pressure that the roller exerts on the ring while the milling roller is worn down. Since the pressure should remain constant during operation of the mill, such devices require a complicated regulation and expensive gearings for controlling the drive speed and for constant surveillance of the running frequency.